1. Field of the Invention
The present invention relates to digital data transmission, and more particularly to digital data decoders.
2. Description of the Related Art
Reference is made to U.S. Pat. Nos. 5,838,672 to Ranta and 5,479,444 to Malkamaki et al and “Mobile Radio Communications” published by John Wiley & Sons, Raymond Steele (Ed.) for a description of the prior art and technological background.
The following description is based on the GSM cellular communications system for which the invention is of particular utility. It will be apparent to those skilled in the art, however, that the invention may be applied to other systems of digital data transmission.
When a mobile phone terminal is to be used to communicate via a network, it must first obtain synchronization with the network. This is essentially a three step passive process. The mobile terminal must synchronize with the base station transmission in time, then frequency and then must read control information to enable the location updating procedure. In the following description it is assumed that a channel containing a broadcast control channel (BCCH) has been chosen.
The channel estimation method according to a prior art arrangements is shown in the flow chart of FIG. 1. At step 401, initially a Frequency Correction Burst (FCB), which is an unmodulated carrier, is sought by scanning of the wanted channel. When a FCB has been received the burst is used to provide coarse time and frequency synchronizations e.g. by means of a narrowband filter at step 402. The coarse time and frequency synchronizations are applied to the next stage of the synchronization process.
When a Synchronization Burst (SB) is received at step 403, it is processed and used to refine both time and frequency synchronization at step 404, 405. The SB contains channel coded information which enables the mobile terminal to access the network. After successfully decoding the SB the mobile terminal is fully synchronized to the network and communications can proceed.
The current arrangements for Synchronization Burst equalization are limited in performance by their intolerance to residual frequency offsets arising from the estimate of the frequency derived from the Frequency Burst. What this means in practice is that when a noisy estimate of the frequency offset is derived from the Frequency Burst processing, then the probability of successful decoding of the Synchronization burst is significantly reduced. Under these conditions the mobile terminal is likely to fail to synchronize with the network.
Existing arrangements for Synchronization Burst decoding are based on the conventional techniques of channel estimation, equalization and convolutional decoding. The channel estimation technique used depends upon calculating the cross-correlation of the expected 64 symbol training sequence with the 64 symbol training sequence received in the synchronization burst. This cross Correlation gives an estimate of the propagation channel. For a channel where no frequency error is present such existing arrangements provide a satisfactory channel estimate.
This is typically not the case, however, on a real fading channel when the mobile terminal is trying to gain initial synchronization with the network. For a fading channel a residual frequency error is carried over from the imperfect frequency estimation derived by the initial Frequency Burst detection. A residual frequency error on the received symbols of the Synchronization Burst is manifest as a constant, accumulating phase offset as a function of time.
This in turn affects the channel estimate obtained by performing a cross correlation of the received Synchronization Burst symbols with the expected training sequence so as to cause a degradation of the equalization procedure. When the bit error rate for the equalized burst exceeds that which can successfully be tolerated by the channel coding used to protect the information contained in the SB, then the synchronization will fail.